One type of accelerator allowing the acceleration of high-energy particles is the cyclotron. The cyclotron accelerates charged particles—for example protons—moving in an axial magnetic field and along a spiral trajectory, by applying a radiofrequency alternating voltage (also called an RF voltage) to one or more acceleration electrodes (sometimes also called “dees”) contained in a vacuum chamber. This RF voltage produces an accelerating electric field in the space which separates the dees, thereby making it possible to accelerate the charged particles.
As the particles accelerate, their mass increases because of the relativistic effects. Accelerated in a uniform magnetic field, the particles therefore shift progressively out of phase with respect to the radiofrequency accelerating electric field.
In practice, two techniques are used to compensate for this phase shift: the isochronous cyclotron and the synchrocyclotron.
In a synchrocyclotron, the intensity of the magnetic field decreases slightly with radius so as to ensure correct focusing of the beam, and the frequency of the RF voltage is progressively decreased so as to compensate for the relativistic gain in mass of the accelerated particles as the radius of their trajectory increases. In this case, the frequency of the RF voltage must therefore be modulated cyclically over time: it must decrease in a constant manner during an acceleration phase between the capture and the extraction of a packet of particles, and then it must increase rapidly so as to be able to accelerate the next packet, and so on and so forth in a cyclic manner for each packet of particles.
The RF device of a synchrocyclotron thus typically comprises an accelerating electrode linked by a transmission line to a variable capacitor (sometimes also called a “RotCo”). This assembly forms a resonating RLC circuit, whose resonant frequency will vary as a function of the value of the variable capacitor. This type of variable capacitor typically comprises a rotor having moveable electrodes and a stator having fixed electrodes. When the rotor is set rotating, the moveable electrodes position themselves in a cyclic manner facing the fixed electrodes, thereby producing a cyclic variation of the capacitance as a function of time.
Such RF devices are for example known from patents GB655271 and WO2009073480 which fairly briefly disclose a Rotco.
K. A. Bajcher et al. of the Joint Institute for Nuclear Research in Dubna have pondered various problems related to this known design of Rotcos (K. A. Bajcher, V. I. Danilov, I. B. Enchevich, B. N. Marchenko, I. Kh. Nozdrin and G. I. Selivanov: Improvement in the operational reliability of the 680 MeV synchrocyclotron as a result of the modernisation of its RF system, Report 9-6218, Dubna, 1972).
One of the problems that they mention is the degradation of the sliding electrical contacts between the rotor and the conducting enclosure, possibly leading to poor operation, or indeed to a complete breakdown of the RF device. Another problem, which is in fact one of the consequences of the degradation of these contacts, is the degradation by electro-corrosion of the bearings which support and guide, in rotation, the shaft of the rotor.
Mints et al., in “Radio-frequency system for the 680 MEV proton synchrocyclotron” (Institute for Nuclear Research, USSR, page 423, FIGS. 4 and 5) proposes an RF device in which an additional coaxial capacitor (reference 5) is placed electrically in parallel with the bearings so as to reduce the RF currents passing through said bearings. Each bearing is moreover protected by a bronze sliding contact between a fixed part and a moveable part of the bearing. These bearings nonetheless continuing to be traversed by high RF currents, this does not satisfactorily solve the problems mentioned hereinabove.
These problems are accentuated by the fact that the RF devices for synchrocyclotrons which are undergoing development are of higher power and that their Rotcos will have to be capable of conducting RF currents of possibly up to for example 1000 A, under voltages of possibly up to for example 18000 V. The rotor will also revolve at higher speeds of possibly up to for example 7000 revolutions per minute.
These problems are moreover still topical, as attested more recently by A. Garonna in his paper “Synchrocyclotron preliminary design for a dual hardontherapy center” (MOPEC 042, conference IPAC'10—May 2010—Kyoto Japan, page 554 “frequency modulation”—second paragraph). It is proposed therein to remedy the problems mentioned by utilizing electronic modulation of the RF frequency.